1. Field of the Invention
This invention relates to a brake system, particularly a hydraulic brake system with enhanced braking force for the rear wheels.
2. Description of the Prior Art
A conventional hydraulic brake system includes hydraulic lines, a brake booster for amplifying a depression force applied to a brake pedal and a master cylinder for converting an output from the brake booster to hydraulic pressure so that the vehicle is decelerated in accordance with the depression force applied to the brake pedal. Two hydraulic lines are usually utilized for almost all vehicles in order to transmit the hydraulic pressures from the master cylinder to the wheel cylinders. A tandem master cylinder is a known device for supplying pressure to the two hydraulic lines. Further, a vacuum booster and a hydraulic booster can be employed for amplifying the depression force applied to the brake pedal.
Japanese Laid-Open Patent Publication No. 64(1989)-47659 discloses a hydraulic brake system having a hydraulic brake booster. The hydraulic brake booster includes a modulator valve for modulating hydraulic pressure supplied from a pressure source and for supplying the modulated pressure to one of the two lines.
In this conventional hydraulic brake system, sufficient braking force can be obtained if the pressure source fails. When the booster fails due to a lack of pressure from the pressure source, both front and rear wheel cylinders are connected to the master cylinder so that sufficient hydraulic pressure is supplied to all of the wheel cylinders from the master cylinder.
Further, it is well known in the art that an anti-lock control apparatus can be adapted to the hydraulic brake system. Under anti-lock control, slip or skid of each wheel can be effectively reduced so that braking safety is increased.
In the conventional hydraulic brake system, however, the braking forces of the rear wheels are designed to be smaller than the braking forces of the front wheels because the load on the front wheels is generally heavier than on the rear wheels and deceleration of the vehicle reduces the load on the rear wheels. As a result, the brake pads associated with the front wheels are worn out in a short period.